Harold is a special and important character in the Fallout story, who originated from Vault 29, which he left in 2090, and lived in the Hub's Oldtown around 2161, in Los around 2208, in Gecko around 2241, and in the Oasis around 2277, and thus met the Vault Dweller, the Chosen One and the Lone Wanderer. He is one of the few characters to appear in all 3 major Fallout installments. He is horribly mutated and looks more or less like a regular ghoul, though he's actually just a human who was affected by FEV exposure (in a military base).

Harold eventually found himself overwhelmed by Bob, and became rooted to the ground during his travels in the Capital Wasteland region. Here he was discovered by several people who began to worship him as a god, and a small and exclusive cult known as the Treeminders began to form in secrecy. Bob began to blossom and many plants grew in this area, which became green with life, a stark contrast to the outer wasteland. During this period, Harold developed an alarmingly powerful mutation which enabled him to "see" the area around Oasis, through the very trees that came from Herbert/Bob's seeds.

He would go on to use this ability to bring in wanderers, all the while hoping one of them would put an end to his troubles- unfortunately, the Treeminders strange ways and insistence on using an intoxicating, potentially poisonous "sap" (as a "purification" ceremony) drove off most of them, many in worse mental shape than they were prior. His persistence would pay off when the Lone Wanderer came into view- on his orders, the Treeminders allowed the wanderer safe passage into Oasis.

Harold revealed his true identity to the Wanderer, and begged for mercy; after being stuck in the same position for decades he was eager for death. The other dwellers of "Oasis" either ignored or, in their religious fanaticism, interpreted Harold's wishes as a moral and spiritual test, much to Harold's chagrin- thus his only recourse was to ask the Lone Wanderer to take his life. However, two members of the Treeminders (namely, the leader and his wife) are found arguing about Harold. The husband wishes for Harold's gift of plant life to be kept safe and confined to Oasis, while his wife wishes for Harold's gift to be shared and spread throughout the wasteland.

They each ask the Lone Wanderer to accomplish their own wishes, and the Lone Wanderer must decide if he will carry out Harold's wish of death, the husband's wish of keeping Harold to just the Oasis, or the wife's wish and spreading his glory to the wasteland. A fourth, more negative option of setting him on fire also exists- this will draw hostility from all the Treeminders. Through granting Harold's wish, the Lone Wanderer's skin will be as hard as Harold's. If granting the optional wishes, Harold will be spoken to through a variety of dialogue, one of them being that he has been given a gift and must stay alive to save the lives of others. Harold, finding a new reason to live, replies that he was selfish to want to kill himself, and then asks Herbert/Bob if he agrees.

To the untrained eye, Harold appears to be an ordinary ghoul. This is not so, however. Ghouls are the result of massive radiation damage to a human body: Harold is a product of the Forced Evolutionary Virus. Unlike most people who are exposed to FEV, he did not become a super mutant, but is the result of a unique combination of radiation damage from constant low-level exposure, indirect exposure to FEV (It is unclear how Harold was infected with FEV as he blacked out for a while) and a fair amount of random chance. He's not a ghoul, and he's certainly not a super mutant. To quote Tim Cain, "Harold is special."

Furthermore, Tim Cain has this to say on the subject: "As for contact [with FEV], any contact at all will infect the subject, but the amount of contact determines the result. For example, I imagine Harold had some contact with the virus, but he was not fully immersed in it, so he became a different mutant than the Master's subjects. Full immersion, of course, is the preferred method of infection, as it provides the virus a large surface area for infection."

It is interesting to note Harold's unusual behavior in treating Bob as if the plant were actually sentient. Although at no point does the plant indicate that it can convey feelings, Harold certainly is not stupid, and perhaps only crazy in the humorous sense- yet he still insists on hearing out the plant's opinions on certain matters that he might otherwise take seriously. Given Harold's newfound ability to "see" through the trees that are descended from Bob through the use of extreme concentration, it may not be much of a stretch to suggest that both are deeply linked in some unknown way, almost certainly a result from FEV exposure.

Despite his appearance, Harold is a kind-hearted soul with a comical personality. He has made many friends throughout his travels, potentially including the Vault Dweller, the Chosen One, and the Lone Wanderer. He is also good with kids, as evidenced by his friendship with Sapling Yew in Oasis, whom he is particularly fond of.